Testing and listing of electrical materials and processes make sense but the application of NEC® grounding and bonding rules is plagued with loopholes. The criteria for electrical materials used in buildings certainly is not and should not be as critical as that used for the Trident Missile. After all, we don't want to have to spend any more time and money than necessary to achieve a reasonably safe installation - safe from fire and electrical shock. Military Standards, however, do have one underlying rule that does make sense and could be better applied to the building industry. That is the coordinating of standards so that one weak length does not eradicate the application of more critical standards. That is common sense. For instance if the grounding and bonding criteria for one section of a ground fault return path is extremely rigid, but other criteria for sections in the same ground fault path are casual, or non existence, then the entire ground fault path is jeopardized by the weakest section.
This article compares four UL® standards used for ground fault testing, to the 1999 National Electrical Code® requirements to find out if the NEC® is in fact "in the dark" about test criteria coordination. The four UL® standards are UL® 514B for Fittings for conduit boxes and outlet boxes, UL® 467 for electrical grounding and bonding equipment, UL 886 for outlet boxes and fittings for use in hazardous locations, and UL's testing data for cable trays.
The premise wiring system has the greatest ground fault and phase to phase fault current available at the service. To make things worse, the service conductors are protected at the wrong end, the load end. Why this is done is a matter of simple economics. The utility companies are not required to install secondary protection at the utility transformers and rely on the primary protective devices that are commonly set at 300 percent of the transformer's primary full load current. This practice permits a horrendous amount of fault current to flow in the service drop and service entrance or service lateral conductors on the supply side of the service overcurrent protective device. For this reason, the grounding and bonding of service equipment is critical on the supply side of the service. After all, if there is a fault on the supply side of the service enough fault current must flow to open the primary fuses at the utility company's transformer. In many cases this simply does not happen. It has been reported by competent engineers that instead of opening the primary fuse, the service raceway will burn open and in some cases cause a fire.
The Code addresses the critical bonding requirements on the supply side of the service protective device in Section 250-94 that specifically does not allow standard locknuts as the sole bonding means. Bonding means that are permitted include bonding type locknuts and bushings. Also included are threaded couplings and bosses, threadless couplings and connectors, and using the grounded service conductor with exothermic welding, listed pressure connectors, listed clamps, or other listed means as permitted in Section 250-8. Section 250-94 is the Code's most stringent bonding requirements and, with the exception of not using the grounded conductor, is also a requirement for bonding in hazardous locations as ruled in Section 250-100.
UL® Standard 467 is the standard used for
listing grounding and bonding equipment and requires the most critical
short-time current tests of any of the standards discussed here.
The data from Table 14-1 is below.
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Where the UL® 467 test is performed with
a grounding or bonding fitting intended for use with a grounding conductor,
a grounding conductor not less than 2 feet long is used. After the
test the only requirement is that continuity shall exist. Unfortunately,
in many installations the grounding conductors are several feet long or
less than 2 feet. But this problem is just the tip of ice berg as
we shall see.
Conduit Fittings and outlet boxes are tested to UL® 514B. Section 36.1 of this standard requires a resistance test causing a direct current of 30 amperes to flow through the connections between the fitting and the conduit, tubing, cable, box, or enclosure to which the fitting is assembled.
But it doesn't stop here. What about flexible conduit
fittings used in hazardous locations? For these, UL®
886 requires a current of 50 amperes flowing through the connections between
a flexible connection fitting or a flexible fixture fitting and conduit
where the voltage drop shall not exceed 150 millivolts. The flexible
connection also must pass an arcing test described as follows:
"A flexible connection fitting and a flexible fixture fitting shall be subjected to an arcing test in which the fitting is wired with one insulated copper wire sized in accordance with the value specified in the second column of Table 22.1. A 2 inch length of bare copper wire, sized in accordance with the value specified in the third column of Table 22.1, is to be spliced in the insulated conductor. "Cable Tray is only listed for its suitability as an equipment grounding conductor. There is no UL standard, but there are file letters verifying that cable tray couplings are subjected to a 30 ampere test very similar to the resistance test required by UL 514B discussed above."One sample fitting is to be tested with this bare copper wire located in the center of the fitting, and one sample is to be tested with the bare copper wire located near one end of the fitting. A 250-volt, direct-current voltage supply with series resistance in the line to limit the current flow to the value specified in the fourth column of Table 22.1 is to be used. The flexible metal tubing and reinforcing metal braid of the assembly is to be connected to the negative side of the circuit through a fuse, and the circuit is to be energized until arcing and burnout of the bare copper wire occurs. The size of the fuse is to be 6 amperes for 1/2, 3/4, and 1 inch trade size conduit, 15 amperes for 1 1/4 inch trade size conduit, and 30 amperes for the 1 1/2 and 2 inch trade size conduit."
"The grounding fuse shall not open, and the insulating inner liner in the fitting shall not burn through as a result of the test."
Table 22-1 is given below:
Arcing Test Parameters for Flexible Fittings
Conduit Trade Size
Insulated Conductor Size
Bare Copper Wire Size
Current in amperes
1/2 12 18 200 3/4 8 14 400 1 6 12 550 1 1/4 2 10 950 1 1/2 1/0 8 1250 2 4/0 6 1950
Summary:
The diagram below illustrates the lack of coordination
in testing grounding materials. It is obvious that the strict requirements
of UL 467 are compromised by less severe testing of other components that
make up the ground-fault return path. In the case of high resistant grounding
these components may be part of the phase to phase short-circuit path.